The present invention relates to a method and to apparatus for chromatographic analysis of ionic species in solution. More particularly, the invention is directed to an improved method and apparatus by means of which chromatographic analysis may be conducted without the need for using a suppressor device to suppress the high background conductivity of eluants used in the analytical procedure.
Apparatus and methods for quantitatively analyzing ionic species utilizing liquid column chromatography are known in the art. Specific apparatus and techniques are described in H. Small et al., U.S. Pat. No. 3,920,397, issued Nov. 18, 1975, and T. S. Stevens et al., U.S. Pat. No. 4,474,664, issued Oct. 2, 1984 the entire disclosure of each of these patents is hereby specifically incorporated herein by reference, to the extent it is not inconsistent herewith.
The apparatus of the prior art has included the following distinct components as essential elements in the analytical method:
a. A chromatographic column. PA0 b. An ion separator exchange resin supported in the column. PA0 c. A suppressor device such as a column or hollow fiber device. PA0 d. An eluant or developing agent, and PA0 e. A detector and indicator such as a conductivity cell to monitor the effluent from the column and to identify the ionic species emanating therefrom.
The general procedure includes the steps of introducing into the column a solution containing the ions to be identified and analyzed. An eluant or developer solution is then added to the column to effect a spacial and time-correlated separation of the various ionic species as the latter traverse the resin bed. In order to prevent the eluant or developing reagent from interfering electrically with the detection of the ionic species to be analyzed, it has been the common practice to rely upon a second resin bed or hollow fiber suppressor to convert that reagent into weakly ionized (low conductivity products) without substantially interfering with the separation of the ionic species to be analyzed and passing through the column, or without significantly reducing their conductance.
Since its development, ion chromatography has proven to be a highly useful technique for the analysis of ions. A factor contributing to the usefulness of this technique is the capability of relying upon conductivity as a means for detecting the ions in the column effluent. The method itself has exhibited a broad universality, a high degree of sensitivity and speed, particularly in the analysis of inorganic ions. However, as heretofore practiced, despite the advantages indicated above, and others, the method itself suffers from several objectionable features. In some prior art procedures, objectionable features stem from the need to use a suppressor column for supressing the high background conductivity of the eluant or the developer, so that the sample ions may be detected without substantial interference.
In carrying out the types of analysis described, a prior art technique has been to employ a train of two columns, a separating column followed by a suppressor column. As indicated above, the first column operates to separate and resolve the ions by conventional elution chromatography. The prior art suppressor column functions to strip or to transform the eluant containing a highly conductive displacement ion to a less conductive level. This arrangement permits the ions of interest to be monitored by conductimetric detection. There are several drawbacks inherent in the technique described. A principal objection and inconvenience in many prior art metods is the need for frequent regeneration of the suppressor column, typically every eight to twenty four hours of operation. Other drawbacks are reduced resolution and sensitivity due to the band spreading in the suppressor column. Additionally, ion exclusion effects in the suppressor column can adversely affect ion separation. The suppressor column also results in increased back pressure and extended elution time. In some cases there are interfering or adverse reactions between the ions of interest and the materials contained in the second (the suppressor) column. Many of these objections have been obviated by the use of continuously regenerated membrane devices.
Because it is deemed more desirable to work with as simple a system as possible, there has been a continuing interest in developing a chromatographic analytical scheme which will operate effectively to separate ions without the need for a suppressor device, but which will still permit the use of conductimetric detection techniques.
It is an element of the present invention that there is provided an effective technique for utilizing a single column--the separator column--and no suppressor. While there has been some initial progress in this direction, for example, by applying conductimeric detection directly to the eluant from the ion exchange separator, the sensitivity of such prior art arrangements is compromised by the "noise" in the relatively high background conductivity of the eluant. It is, therefore, a principal aim of the present invention to provide a chromatographic analysis scheme which is effective to separate ions without the use of the suppressor column, and which effectuates the continued utilization of highly sensitive conductimetric detection as the means of ultimate analysis, as may be afforded when working in very low conductivity environments such as pure water.